1. Field of the Invention
The present invention relates to a process for manufacturing a spherical cerium-activated catalyst for the synthesis of ammonia.
More particularly the invention concerns a process in which cerium-activated catalysts in the form of spheres having regular diameters of from 1 to 25 mm are prepared by mixing magnetite with (by weight) 2-3.5% of aluminium oxide, 0.8-2% potassium hydroxide, 2-3.5% calcium oxide, 0.1-0.4% magnesium oxide and 0.2-0.5% silica; melting this mixture in a furnace at a temperature of at least 1600.degree. C.; air-cooling the molten mass, removing melted slag; crushing the deslagged mass in a crusher and pulverising it in a rod-mill; adding in a mixer to the so obtained powder a cerium nitrate solution in quantities to obtain in the final catalyst a metallic cerium concentration of 0.5 to 2.5%; pelletising the so added powder in a tray pelletiser to obtain a sphere shaped catalyst; drying said catalytic spheres in a furnace at 100.degree.-200.degree. C., and sintering them in an argon atmosphere at a temperature of 1250.degree.-1350.degree. C.
The invention also comprises a cerium-activated catalyst in form of pelletised spheres, which contain from 0.5 to 2.5% cerium, show a high activity, a high resistance to thermal stresses and to chemical poisons, a uniform distribution of the synthesis gas and a low pressure drop, and are safe to handle and not-brittle.
2. Description of the Prior Art
It is known from old literature that elements such as ruthenium, cerium, titanium activate iron-based catalysts for ammonia synthesis when skillfully added in small quantities.
Processes have been recently disclosed in which the promoted and reduced iron oxides are treated with an aqueous solution of a cerium nitrate. More particularly the process according to U.S. Pat. No. 3,951,862(Lummus) comprises: mixing an iron oxide with traces of conventional promoters (alumina, calcium oxide, potassium and silica) fusion of the blend in the presence of 0.1-0.2 powdered graphite; crushing and screening of the fused and cooled pig; reduction of the catalyst with H.sub.2 or H.sub.2 +N.sub.2 gas mixture; treatment of the reduced catalyst with a cerium nitrate solution; and drying. In the U.S. Pat. No. 3,992,328 (also to Lummus) the fused and reduced catalyst is submitted to a process in which air is evacuated from the pores before impregnating it with the aqueous solution of a cerium salt or of a mischmetal salt including cerium.
Accordingly the main features of the processes described in the above patents are:
(1) the form of the catalysts is that obtained by breaking the pig with a hammer, by crushing and by screening (the under- and oversized material being recycled); irregular granules of 1-3 mm are used which however are scarcely efficient in certain plants (especially where there is no radial stream); PA1 (2) a first reduction of the catalyst with H.sub.2 or H.sub.2 +N.sub.2 is necessary; PA1 (3) the treatment with the cerium nitrate solution is carried out on the catalyst so reduced; PA1 (4) complex treatments with mixtures of oxygen and nitrogen (1% O.sub.2 and 99% N.sub.2 in a first step, and 5% O.sub.2 and 95% N.sub.2 in a third step) are necessary; PA1 (5) the catalyst undergoes full oxidation by the air and nitrous and nitric oxide formed by the decomposition of the cerium nitrate; PA1 (6) a second reduction is thus necessary; PA1 (7) although it is said that the amount of cerium added to the reduced catalyst may range from 0.1 to 1.5%, effective results are obtained with 0.3-0.8% particularly with 0.45-0.7% by weight of cerium; PA1 (8) the catalysts seem most efficient within a temperature range of from 400.degree. to 480.degree. C.
U.S. Pat. No. 4,073,749 (S.I.R.I) describes a spherical catalyst which is pelletized thanks to the addition of water and bentonite to the catalyst powder, i.e. by the addition of a binder mixture which reduces the catalyst activated mass.